1. Field of the Invention
The present invention relates generally to systems and methods for reducing vibration in electro-mechanically cooled devices, and more particularly of active vibration control for an electro-mechanically cooled device.
2. Discussion of Background Art
High-purity germanium (HPGe) radiation detectors are well known for their performance and reliability characteristics and have become a standard by which other radiation detectors are compared. HPGe detector systems have long been the standard for critical applications including uranium enrichment and plutonium isotopic analysis. Combining this high performance with the additional property of high efficiency, HPGe detectors provide very high resolution radio-nuclide spectra within a minimum acquisition time.
Though HPGe detector systems are recognized as a basic tool for isotopic analysis and have many field applications, since extensive support equipment is required for optimum operation, these systems have historically been used only in laboratories. HPGe detector systems typically require signal processing electronics for data acquisition (shaping amplifier, test pulser), a high voltage power supply for detector bias, a multi-channel analyzer (MCA), a computer including requisite software necessary for spectral analysis, and (most importantly) a continuous supply of Liquid Nitrogen (LN) to maintain the detector at its proper operating temperature. As a result, operating an HPGe detector in the field has been very difficult.
As the need for field deployable high energy resolution detectors has increased, many of the HPGe support systems have been reengineered for portable applications. Several integrated electronic systems are currently available that, when accompanied by a detector and portable computer, provide a portable data acquisition capability. Current systems on the market include the Mini MCA (GBS Elektronik), the M.sup.3 CA (Los Alamos National Laboratory [LANL]), the Inspector (Canberra), and the Dart, MicroNOMAD, and NOMAD (EG&G ORTEC).
In addition to the reduced size, weight, and power requirements of detector systems, several manufacturers have developed small LN dewars that can provide temporary portable operation. These portable dewars typically hold enough LN to allow portable operation from 12 to over 36 hours. LN consumption is approximately 0.5 liters/day/watt with the typical HPGe detector heat load being approximately 2-3 watts once cooled to an operating temperature of about 77 Kelvin.
While these developments have provided significant improvements allowing for limited portable HPGe detector operation, many applications require longer acquisition periods, unattended operation, and/or operate in an elevated temperature environment that would rapidly deplete a typical LN supply. For these more demanding applications, either a larger LN dewars or an Electro-Mechanical Cooling (EMC) system is required.
Development of EMC systems was initially driven by infrared imaging technology for space-based applications. The imaging arrays were typically small volume devices and the space environment provided a good vacuum and a cold environment, so only a minimal amount of cooling was necessary. Since vibration in these applications would be detrimental to image quality, cooling the infrared imaging detector with an absolute minimum of vibration was critical. These cooling systems were often turned off for short periods during image acquisition to further reduce vibration. Several organizations (including TRW, Hughes, and Lockheed) developed methodologies for providing the required cooling, however, for various reasons these space-based solutions have not found widespread use in terrestrial-based instrumentation. For instance, a relatively higher heat load experienced by ground-based detection devices limits the time an EMC cooling system could be de-energized.
Most ground-based EMC-HPGe detectors incorporate continuously operating cooling systems into their design. These mechanical cooling systems inherently generate vibrations. These vibrations degrade the spectral resolution of HPGe detectors. This type of degradation, referred to as "microphonic noise," is defined as electronic noise which results from vibration. For instance, pre-amplifier circuits within HPGe detectors typically consist of a junction field effect transistor (JFET) whose base is connected to a germanium detector crystal by a flexible wire. Vibration from the EMC apparatus cause the wire to vibrate with respect to surrounding electrical components. Since the wire forms a parasitic capacitor with the surrounding components, this vibration induces a voltage on the wire which adds noise to a signal transmitted from the detector. This noise component reduces a signal to noise ratio of the pre-amplifier and degrades overall sensitivity of the EMC-HPGe detection system.
In response, EMC manufacturers have developed vibration control mechanisms that attempt to cancel out mechanical vibration. Some designs are "passive" and merely employ damping mechanisms. Other designs are "active" and use a dynamically adjustable counterbalance.
An active vibration control system typically operates by first characterizing the EMC-HPGe detection system based on well known vibration modeling equations and then using feedback to cancel any system vibrations using the adjustable counterbalance. These active vibration control systems however are "static" in design in that they only characterize the EMC-HPGe detection system once at the factory before shipping them out to the field. Kenneth W. Neufeld, Wayne D. Ruhter, and Eric H. Anderson describe such a statically-characterized active vibration control system in their paper entitled, Interim Cryo-Cooler/Detector Report, dated Apr. 19, 1995.
If the EMC was never moved from its position at the factory where the static characterization occurred, the static characterization method would be acceptable. However, field deployable EMC-HPGe detection systems are by their nature, often moved and placed on a variety of different surfaces and in a variety of different environments. These varying surfaces and environments change the vibration characteristics of the EMC-HPGe detection system from what it was at the factory. Thus, the static characterization which was good at the factory may no longer be acceptable in the field.
In response to the concerns discussed above, what is needed is an system and method of active vibration control for an electro-mechanically cooled device that overcomes the problems of the prior art.